1. Field of the Invention
The present invention relates to a cartridge drop detection device for use in a magnetic tape library apparatus.
2. Description of the Related Art
Cartridges containing magnetic tape recording media are still in widespread use for such reasons as their low cost-per-bit compared with other types of storage media and their large storage capacity. A tape library apparatus for storing cartridges containing magnetic tape recording media holds, for example, 100 to 700 cartridges, and performs read or write operations on a magnetic tape recording medium by retrieving the selected cartridge from a media rack and loading it into the necessary tape library by means of a tape transport robot.
Such a tape library apparatus is used, for example, to back up important data concerning customers. Therefore, any trouble during transport must be avoided as much as possible because it can lead to a serious problem. However, due to vibrations caused by an earthquake or the like, a cartridge may fall off the media rack and drop onto the floor of the tape library apparatus. If the operator, being unaware of the existence of the dropped cartridge, continues to operate the tape library apparatus, the tape transport robot while moving around on the floor may run over the cartridge and destroy it.
In the prior art, an acceleration sensor has been used to detect vibrations caused by an earthquake or the like. Further, the tape transport robot has been equipped with a servo control unit to which a certain magnitude of vibration force is applied externally when vibrations such as those caused by an earthquake occur. Since this external force causes an electric current larger than usual to flow, the servo control unit detects the occurrence of a problem, and stops the operation of the tape transport robot upon detection of the problem.
[Patent Document 1] Japanese Unexamined Patent Publication No. H05-219827
[Patent Document 2] Japanese Unexamined Patent Publication No. H03-91157
In the above prior art method, as abnormal vibrations are detected by relying only on the acceleration sensor or the servo control unit, the operation of the tape transport robot is stopped even when there is no cartridge on the floor of the tape library. This has led to the problem that there are cases where the operation of the tape transport robot is stopped unnecessarily. More specifically, there is no guarantee that a cartridge will necessarily drop whenever an earthquake or the like occurs causing vibrations of the magnitude that exceeds a vibration (acceleration) threshold below which the apparatus can guarantee that any cartridge designed to the specification of the apparatus will not drop. In the prior art, however, the operation of the tape library apparatus has had to be stopped even when all the media stay on the rack.